Welding method, welding system, and welding jig for robotically welding a motorcycle fuel tank

ABSTRACT

A welding jig for holding a motorcycle fuel tank when the fuel tank is welded includes a plurality of attachments for holding said fuel tank through resilient bodies. The plurality of attachments have springs hold the outer plate of a fuel tank, and an inner jig positions an inner plate. The attachments are attached to clamp arms which are opened/closed by cylinders. When the overlap portion between the outer and inner plates is welded, the thermal deformation caused by welding is accommodated by the springs. The attachments are spaced from the fuel tank in the order in which the welding point is approached.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/525,124,filed 18 Feb. 2005, which is the US National Phase of InternationalApplication No. PCT/JP03/10297, filed 13 Aug. 2003, which claimspriority from Japanese Patent Applications nos. 2002-241528 and2002-241535 both filed 22 Aug. 2002. The entire content of each of thepriority applications is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a welding method, a welding system, anda welding jig for welding a fuel tank for a motorcycle, and moreparticularly to a welding method, a welding system, and a welding jigfor reducing the effect of thermal strain caused by welding.

BACKGROUND ART

As shown in FIG. 15 of the accompanying drawings, a fuel tank 200 for amotorcycle generally has an outer side panel 202 and an inner side panel204 having its bottom welded to the outer side panel 202. The outer sidepanel 202 and the inner side panel 204 have respective ends bentdownwardly into flanges 206 that are generally seam-welded.

In motorcycles, a motorcycle having a steering handle at a highposition, and which is operated by a rider whose upper half body is keptsubstantially upright, i.e., a so-called American-type motorcycle, hasits fuel tank 200 regarded as being particularly important in terms ofappearance. For the fuel tank 200 to be aesthetically pleasing, thewelded flanges 206 should preferably not be exposed. Furthermore, sincethe center of gravity of the fuel tank is displaced upwardly by theheight of the flanges 206, the flanges 206 are disadvantageous withrespect to efforts to lower the center of gravity of the motorcycle.

The existence of the flanges 206 also limits the capacity of the fueltank 200.

There has been proposed a fuel tank having a structure in which flangesdo not project downwardly due to seam welding, but rather are bentinwardly (see, for example, Japanese Laid-Open Patent Publication No.10-76985). According to this proposal, however, useless space is presentabove the flanges, which limits the capacity of the fuel tank.

For manufacturing a flangeless fuel tank, a skilled welder is required,in order to weld the fuel tank by means of arc welding or the like. Ifthe fuel tank is automatically welded by a robot, then since the fueltank must be firmly secured in place, the fuel tank tends to crack sincethermal strains cannot be relieved during welding, resulting in areduced yield. In such a case, when the fuel tank cracks, it still needsto be repaired by a skilled welding operator.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems. It isan object of the present invention to provide a welding method, awelding system, and a welding jig for reducing the effect of thermalstrain caused by welding when a fuel tank for a motorcycle is welded,thereby preventing the fuel tank from cracking.

According to the present invention, there is provided a welding methodfor welding a motorcycle fuel tank, using a plurality of opening/closingmechanisms having one or more attachments, for bringing the attachmentsinto and out of abutting engagement with the fuel tank, and comprisingthe steps of bringing the attachments into abutting engagement with thefuel tank with the opening/closing mechanisms, thereby holding the fueltank, and bringing the attachments out of abutting engagement with thefuel tank with the opening/closing mechanisms in the order in which theattachments are approached by a welded spot where the fuel tank iswelded, when the welded spot moves along a welding line.

When the welded spot moves along the welding line, the attachments arebrought out of abutting engagement with the fuel tank in the order inwhich the attachments are approached by the welded spot. Therefore, theeffect of thermal strain, caused by welding the fuel tank is reduced,preventing the fuel tank from cracking.

If the attachments hold the fuel tank through resilient bodies, theeffect of thermal strain caused by welding the fuel tank can be reducedfurther, even while the attachments are kept in abutting engagement withthe fuel tank.

The fuel tank may have an outer side panel having inwardly extendedends, wherein an inner side panel is welded to the outer side panel. Inthis case, the attachments hold the fuel tank while outer surfaces ofthe ends of the outer side panel and inner surfaces of ends of the innerside panel are superposed, or while the ends of the outer side panel andthe ends of the inner side panel are in abutment against each other.With the fuel tank being thus held, the inner side panel and theouter-side panel are accurately positioned. Therefore, the welded fueltank can have a shape that is free of flanges.

The attachments are brought out of abutting engagement with the fueltank by the opening/closing mechanisms when the welded spot reaches apoint which is spaced 20 mm or less from a reference point on thewelding line which is closest to an abutment point where each of theattachments and the fuel tank abut against each other.

According to the present invention, there is also provided a weldingsystem for welding a motorcycle fuel tank, which has an outer side panelhaving inwardly extended ends, and an inner side panel welded to theouter side panel, comprising a plurality of opening/closing mechanismshaving one or more attachments, the opening/closing mechanisms bringingthe attachments into and out of abutting engagement with the fuel tank,a welding machine automatically operable for welding the fuel tank, anda controller connected to the opening/closing mechanisms and the weldingmachine, wherein the controller controls the opening/closing mechanismsto bring the attachments into abutting engagement with the fuel tank tohold the fuel tank, and thereafter, the controller determines theposition of a welded spot where the fuel tank is welded by the weldingmachine and the positions of the attachments, and if the relativeposition of the welded spot with respect to each of the attachmentssatisfies a predetermined standard, the controller controls at least oneof the opening/closing mechanisms to bring the attachments out ofabutting engagement with the fuel tank.

According to the present invention, there is further provided a weldingjig for holding a motorcycle fuel tank when the fuel tank is welded,comprising a plurality of attachments for holding the fuel tank throughresilient bodies.

Since the attachments hold the fuel tank through resilient bodies, theeffect of thermal strain caused by welding the fuel tank can be reduced,thereby preventing the fuel tank from cracking.

The attachments are mounted on respective arms having respectiveopening/closing mechanisms. When the arms are fully opened, they areopened wide enough to allow the fuel tank to be attached and detached.When the arms are fully closed, the arms are positioned by respectivestoppers to hold the fuel tank with the attachments.

With the above arrangement, the fuel tank can easily be attached to anddetached from the welding jig, with the attachments being positionedaccurately by the stoppers.

The attachments have pressing force adjusters for adjusting a pressingforce with which the fuel tank is held. The pressing force adjusters arecapable of adjusting the pressing force with which the attachmentscontact the fuel tank, thereby adjusting the allowable amount of thermalstrain caused by welding the fuel tank.

The welding jig may further comprise an outer jig for supporting anouter side panel of the fuel tank, and an inner jig for supporting aninner side panel of the fuel tank, wherein the attachments are providedin the outer jig, and hold side portions and/or end portions of theouter side panel. With this arrangement, the inner and outer side panelsof the fuel tank are held and accurately positioned respectively by theinner and outer jigs.

The outer side panel of the fuel tank has inwardly extended ends,wherein the attachments hold the fuel tank while outer surfaces of endsof the outer side panel and inner surfaces of ends of the inner sidepanel are superposed, or while the ends of the outer side panel and theends of the inner side panel are in abutment against each other. Whenthe outer side panel and the inner side panel are held in this manner,the fuel tank may be of a shape that is free of flanges.

The welding jig may further comprise a positioning mechanism, which isinserted into a fuel inlet defined in an upper surface of the fuel tank,for contacting the inner portion of the fuel tank to hold the fuel tank.The positioning mechanism allows the fuel tank to be quickly andaccurately positioned with respect to the welding jig.

Each of the attachments has a distal end abutting against the fuel tank,wherein the distal end has a tilting mechanism tiltable in anydirection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a welding system according to anembodiment of the present invention;

FIG. 2 is a perspective view of a welding jig according to theembodiment of the present invention together with a motorcycle fueltank;

FIG. 3 is a front elevational view, partly in cross section, of thewelding jig according to the embodiment of the present inventiontogether with the motorcycle fuel tank;

FIG. 4 is a side elevational view, partly omitted from illustration, ofthe welding jig according to the embodiment of the present inventiontogether with the motorcycle fuel tank;

FIG. 5 is a front elevational view of a cylinder, a clamp arm,attachments, and surrounding parts;

FIG. 6 is a cross-sectional view of an attachment;

FIG. 7 is a front elevational view of a positioning mechanism in acondition in which hooks thereof are elevated;

FIG. 8 is a front elevational view of the positioning mechanism when thehooks thereof are lowered;

FIG. 9 is a perspective view of an inner jig, an inner side panel, andan outer side panel;

FIG. 10 is a block diagram of a controller;

FIG. 11 is a flowchart of a welding process performed using the weldingsystem according to the embodiment of the present invention;

FIG. 12 is a schematic view showing a welding path;

FIG. 13 is a schematic view showing the manner in which the outer sidepanel is welded while it is being pressed by an attachment;

FIG. 14 is a schematic view showing a welding line, an abutment point, areference point, and a welding point; and

FIG. 15 is a perspective view of a fuel tank having flanges.

DETAILED DESCRIPTION INCLUDING BEST MODE FOR CARRYING OUT THE INVENTION

An exemplary embodiment of the present invention will be described belowwith reference to FIGS. 1 through 14. A welding method, a welding system10, and a welding jig 10 a according to the embodiment serve to weld afuel tank 12 for a motorcycle.

As shown in FIG. 1, the welding system 10 has a welding jig 10 a (seeFIG. 2) for holding the fuel tank 12, a jig robot 10 b for setting thefuel tank 12 held by the welding jig 10 a at a predetermined position, awelding robot (welding machine) 10 c for performing a welding process,and a controller 10 d. The controller 10 d is connected to cylinders 28(see FIG. 2) of the welding jig 10 a, a positioning mechanism 44 (seeFIG. 7), the jig robot 10 b, and the welding robot 10 c, for controllingthe welding system 10 in its entirety.

As shown in FIG. 2, the welding jig 10 a is a jig for fixing an outerside panel 14 and an inner side panel 16 of the motorcycle fuel tank 12when the outer side panel 14 and the inner side panel 16 are welded toeach other. The welding jig 10 a has an outer jig 18 for supporting theouter side panel 14 and an inner jig 20 for supporting the inner sidepanel 16. As shown in FIG. 2, the fuel tank 12 has its right sidepositioned forwardly (closer to the steering handle of the motorcycle)and its left side positioned rearwardly (closer to the seat of themotorcycle).

The outer jig 18 is mounted on the tip end of the jig robot 10 b (seeFIG. 1) and is set at a predetermined position for the welding process.The welding process is performed by the welding robot 10 c. The jigrobot 10 b, the welding robot 10 c, the cylinder 28, and so forth, arecontrolled by the controller 10 d.

The outer side panel 14 of the fuel tank 12 has a lower portion in FIG.2 supported by a plurality of lower supports 22 and side and endportions supported by a plurality of attachments 24. Each group of twoor three of the attachments 24 is mounted on a single clamp arm (arm)26. There are a total of eight clamp arms 26, four in each of left andright symmetrical arrays, which are individually openable and closableby cylinders 28.

As shown in FIGS. 3 and 4, the outer side panel 14 has ends 15 extendedinwardly, and the inner side panel 16 has ends whose inner surfaces aresuperposed on the outer surfaces of the ends of the outer side panel 14.A contact region 120 (see FIG. 13) where the ends of the outer sidepanel 14 and the inner side panel 16 are superposed is welded by thewelding robot 10 c, thereby performing one-side welding of a filletjoint. The fuel tank has a fuel inlet 36 disposed in a lower portion ofthe outer side panel 14.

The outer jig 18 has two vertical frames 38 extending rearwardly from aposition substantially beneath the fuel inlet 36, at a substantiallyconstant spaced interval from the lower surface of the outer side panel14, and four auxiliary frames 40 extending laterally, obliquelyforwardly, and obliquely rearwardly, on each of left and right sidesthereof. A plurality of lower supports 22 for supporting the outer sidepanel 14 from below are mounted on the upper surfaces of the auxiliaryframes 40. Rear end support members 42 for supporting lateral sides ofthe rear end of the outer side panel 14 are mounted on the rear ends ofthe vertical frames 38. The lower supports 22 and the rear end supportmembers 42 may be made of a synthetic resin, such as nylon or the like.

A positioning mechanism 44, which is inserted into the fuel inlet 36 forsupporting the fuel tank 12 from inside thereof, is mounted on the frontends of the two vertical frames 38.

As shown in FIG. 5, a base plate 46 and one of the cylinders 28 foropening and closing the clamp arms 26 are mounted on the distal end ofeach of the auxiliary frames 40. The base plate 46 and the cylinder 28make up an opening/closing mechanism 47. The cylinder 28 has a cylindertube 28 a supported by a shaft on the base plate 46 for swingingmovement. The cylinder 28 extends and contracts a rod 28 b to open andclose the clamp arm 26.

The clamp arm 26 has a lower portion supported for swinging movement onan upper shaft 46 a on the base plate 46. The clamp arm 26 has aprotrusion 26 a projecting slightly outwardly from the lower portionthereof and supported by a shaft on the distal end of the rod 28 b ofthe cylinder 28. The clamp arm 26 also has a stopper 26 b projectingslightly inwardly from the lower portion thereof. When the clamp arm 26is closed, the stopper 26 b abuts against an upper surface of the baseplate 46, thereby positioning the clamp arm 26.

The clamp arm 26, when it is closed by the opening/closing mechanism 47,has a first arm member 26 c extending upwardly from the shaft 46 a, asecond arm member 26 d mounted on the distal end of the first arm member26 c and inclined slightly inwardly, and a third arm member 26 e mountedon the distal end of the second arm member 26 d and inclined moreinwardly than the second arm member 26 d. With this structure, when theclamp arm 26 is closed, the clamp arm 26 is spaced at a substantiallyconstant interval from the outer side panel 14. The third arm member 26e may be dispensed with, depending on the location (see FIG. 4).

The first arm member 26 c, the second arm member 26 d, and the third armmember 26 e have respective holes 48 defined therein for installingrespective attachments 24. Two or three attachments 24 are mounted oneach clamp arm 26.

As shown in FIG. 6, each of the attachments 24 comprises a sleeve 50mounted in a hole 48 in the clamp arm 26, an attachment shaft 54 movablein and along a central bore 52 of the sleeve 50, a spring bearing plate56 mounted on the distal end of the attachment shaft 54, an adjustmentnut 60 (pressing force adjuster) threaded over screw threads 58 whichare formed on the outer circumferential surface of the distal end of thesleeve 50, a washer 62 held against the adjustment nut 60, and a spring(resilient body) 64 disposed between the spring bearing plate 56 and thewasher 62.

A ball 66, and an abutment (tilting mechanism) 68, which is tiltable inany direction while being held in sliding contact with the ball 66, aremounted on the distal end of the spring bearing plate 56. The abutment68 comprises two components 68 a, 68 b sandwiching the ball 66therebetween.

A fixing nut 72 is threaded over screw threads 70 that are formed on theouter circumferential surface of the rear end of the sleeve 50. Theclamp arm 26 is clamped in position between an annular flange 74, whichis mounted substantially centrally on the sleeve 50 and the fixing nut72. One or more annular shims 76, for adjusting the length by which theattachment 24 projects with respect to the clamp arm 26 when necessary,are inserted between the annular flange 74 and the clamp arm 26.

A cylindrical bushing 78 having a lubricating function is inserted inthe bore of the sleeve 50. The attachment shaft 54 is smoothly movablewith respect to the bushing 78.

The attachment shaft 54 has a smaller diameter externally threadedportion 80 formed on the rear portion thereof, wherein a knob 82(pressing force adjuster) and an end stopper 84 (pressing forceadjuster) are threaded over the externally threaded portion 80. When theknob 82 and the end stopper 84 are turned, the compression of the spring64 and the projection of the attachment shaft 54 can be adjusted. Aftersuch an adjustment, the knob 82 and the end stopper 84 are fixed inposition as double nuts that are tightened against each other.

When the adjustment nut 60 is turned, the compression of the spring 64can be adjusted. Specifically, the degree of compression of the spring64 is adjustable by means of the knob 82, the end stopper 84, and theadjustment nut 60. Actually, knob 82 and the end stopper 84 are turnedto roughly adjust the compression of the spring 64, and the adjustmentnut 60 is turned to finely adjust the compression of the spring 64.

When the distal end of the abutment 68 is pushed by the outer side panel14, the attachment shaft 54 is moved toward the rear end thereof. Atthis time, the attachment shaft 54 compresses the spring 64, and moves acertain distance depending on the repulsive force of the spring 64.

As shown in FIG. 7, the positioning mechanism 44 is disposed beneath thefuel inlet 36, at an intermediate location between the two verticalframes 38. The positioning mechanism 44 has an upper portion inserted inthe fuel inlet 36. The positioning mechanism 44 has an insert member 88fixed to a frame body 86, a movable member 92 vertically movable by arod 90, and two hooks 94 supported by shafts on the movable member 92for being tiltable slightly outwardly when the movable member 92 islowered. The insert member 88 has a horizontal width D, which isslightly smaller than the inside diameter of the fuel inlet 36.

The two hooks 94 comprise upwardly extending plates shaped bilaterallysymmetrically. Each of the hooks 94 has an upper tooth 94 a projectingslightly outwardly, a longitudinally oblong hole 94 b, and a lower swinghole 94 c. The oblong hole 94 b has a lower portion bent slightlyoutwardly.

Two bilaterally symmetrical fixed support shafts 88 a project from theinsert member 88 at a substantially central height thereof. Twobilaterally symmetrical movable support shafts 92 a project from anupper portion of the movable member 92. As shown in FIG. 7, the fixedsupport shafts 88 a and the movable support shafts 92 a project towardthe direction of the viewer. The fixed support shafts 88 a are insertedin the oblong holes 94 b defined in the hooks 94, and the movablesupport shafts 92 a are fitted in the swing holes 94 c defined in thehooks 94.

For placing the outer side panel 14 on the outer jig 18, the rod 90 ismoved upwardly by a cylinder (not shown). At this time, the two hooks 94are tilted inwardly and positioned within the horizontal width D of theinsert member 88. When the outer side panel 14 is placed on the outerjig 18, an upper portion of the insert member 88 and upper portions ofthe hooks 94 are inserted into the fuel inlet 36.

As shown in FIG. 8, when the rod 90 is lowered, the movable member 92and the hooks 94 are also lowered. The hooks 94 are guided by the fixedsupport shafts 88 a that are inserted in the oblong holes 94 b, and aretilted outwardly. The teeth 94 a of the hooks 94 project beyond theinside diameter of the fuel inlet 36. Upon further descent of the hooks94, the teeth 94 a of the hooks 94 abut against an end of the fuel inlet36, thus holding the outer side panel 14.

As shown in FIG. 9, the inner jig 20 has an elongate upper plate 100, anextension rod 102 projecting from a rear end of the upper plate 100,grips 104 mounted on an upper surface of the upper plate 100, and aplurality of nylon presser plates 106 fixed to a lower surface of theupper plate 100 and matching the shape of the inner side panel 16. Jointlevers 108 are mounted respectively on a rear end of the extension rod102 and on the foremost one 106 a of the presser plates 106. Each of thepresser plates 106 is of a bilaterally symmetrical shape, projectingfrom the upper plate 100 at its center, and has lateral end surfaces ora lower surface matching the shape of the inner side panel 16. The jointlevers 108 respectively engage with joint hooks 110 (see FIG. 2), whichare mounted on front and rear ends of the outer jig 18.

As shown in FIG. 10, the controller 10 d has a welding robot controlunit 130 for controlling the welding robot 10 c, a jig robot controlunit 132 for controlling the jig robot 10 b, a positioning control unit134 for vertically moving the rod 90 of the positioning mechanism 44,and first through eighth cylinder control units 136, 138, 140, 142, 144,146, 148, 150 for controlling the eight cylinders 28, respectively. Thewelding robot control unit 130 and the jig robot control unit 132control respective motor drivers, not shown, which operate the weldingrobot 10 c and the jig robot 10 b. The welding robot control unit 130and the jig robot control unit 132 can detect attitudes of the weldingrobot 10 c and the jig robot 10 b, as well as the positions and speedsof various parts thereof, through feedback signals. Pneumatic valves,not shown, are disposed between the positioning control unit 134 and thepositioning mechanism 44, and also between the first through eighthcylinder control units 136 through 150 and the cylinders 28, whereinsuch valves function to operate the rod 90 and the cylinders 28.

The controller 10 d also has a welding control unit 152 as a maincontrol unit, including a welding point determining unit 154 fortransferring data to and from the welding robot control unit 130 and thejig robot control unit 132. The welding point determining unit 154 givescontrol instructions to the first through eighth cylinder control units136 through 150.

A process of welding the outer side panel 14 and the inner side panel 16of the fuel tank 12 using the welding system 10 and the welding jig 10 awill be described below with reference to FIGS. 11 through 14. Theprocedure to be described below basically is performed by the controller10 d, with certain setting operations being carried out by the operator.

Two welding cycles are performed on left and right sides, as indicatedby points Q1 through Q6, and Q7 through Q12, which as shown in FIG. 12,represent a welding sequence. The points Q1 through Q12 are points onwelding lines V. Point Q1 is a starting point, point Q6 a pausing point,point Q7 a resuming point, and point Q12 an ending point. Points Q2through Q6 and points Q8 through Q11 are reference points on the weldinglines V, which are closest to abutment points P (see FIGS. 13 and 14)where the attachments 24 on the clamp arms 26 disposed closely to thosepoints and the fuel tank 12 abut against each other.

The welding process is started from the foremost point Q1 on the centralline of the fuel tank 12, and is performed progressively through pointsQ2 to Q6. Point Q6 is the rearmost point on the central line of the fueltank 12. After the welding process has been performed up to point Q6,the welding process is interrupted and then goes to point Q7. Point Q7is a point near point Q1 and is set at a location where lap welding canbe performed. The welding process is started again from point Q7, and isperformed progressively through points Q7 to Q12. Point Q12 is a pointnear point Q7 and is set at a location where lap welding can beperformed. The welding process is carried out along the above path bythe welding robot 10 c (see FIG. 1), and may be performed in cooperationwith the jig robot 10 b (see FIG. 1).

Specifically, in step S1 shown in FIG. 11, the clamp arms 26 of theouter jig 18 are opened (see FIG. 5), and the rod 90 (see FIG. 7) andthe hooks 94 are lifted. Then, the operator places the outer side panel14 of the fuel tank 12 on the lower supports 22 with the fuel inlet 36oriented downwardly. At this time, the outer side panel 14 is placed onthe lower supports 22, while the insert member 88 of the positioningmechanism 44 is inserted into the fuel inlet 36. Since the hooks 94, asthey are lifted, are set to a width smaller than the inside diameter ofthe fuel inlet 36, the hooks 94 are kept out of interference with thefuel inlet 36. Inasmuch as the horizontal width D of the insert member88 is slightly smaller than the inside diameter of the fuel inlet 36,the upper portion of the insert member 88 can be inserted into the fuelinlet 36, thereby simply and accurately positioning the outer side panel14 with respect to the outer jig 18.

In step S2, the rod 90 of the positioning mechanism 44 is lowered,thereby lowering the movable member 92 and the two hooks 94 (see FIG.8). As the two hooks 94 are lowered, they are tilted outwardly until theteeth 94 a abut against the end of the fuel inlet 36. The outer sidepanel 14 is now firmly secured with respect to the outer jig 18.

In step S3, the inner side panel 16 of the fuel tank 12 is placed on anupper portion of the outer side panel 14. At this time, the inner sidepanel 16 is placed on the upper portion of the outer side panel 14, suchthat the inwardly extended ends 15 of the outer side panel 14 and theperipheral ends of the inner side panel 16 are substantially superposedon each other. Thereafter, the inner jig 20 is placed on an upperportion of the inner side panel 16.

In step S4, the eight cylinders 28 are actuated to close the clamp arms26, until the stoppers 26 b are brought into abutment against the uppersurfaces of the base plates 46. When the clamp arms 26 are closed, theabutments 68, which serve as distal ends of the respective attachments24, abut against the outer side panel 14. At this time, the abutments 68abut against the outer side panel 14 while compressing the springs 64,and the outer side panel 14 is pressed under a pressing force dependingon the degree of compression of the springs 64. The pressing force canbe adjusted when the adjustment nuts 60 or the knobs 82 are turned, andtherefore may be adjusted to an appropriate level in advance by anoperator.

When the abutments 68 of the attachments 24 hold the outer side panel14, the outer side panel 14 is set in position, and hence a flexed statethat would otherwise occur due to gravity can be corrected. Since theeight clamp arms 26 are disposed four each, in left and rightsymmetrical arrays, they hold the fuel tank 12 in a well balanced state.

Because the abutments 68 have a structure that permits tilting about theballs 66, the distal end faces of the abutments 68 are not held inlocalized abutment, but rather are reliably held in full abutmentagainst the outer side panel 14.

In step S5, the operator brings the joint levers 108 on the oppositeends of the inner jig 20 into engagement with the joint hooks 110. Sincethe presser plates 106 have shapes that match the inner side panel 16,the inner side panel 16 is accurately positioned and fixed with respectto the outer side panel 14.

In step S6, the welding robot 10 c commences a welding operation, forwelding the outer side panel 14 and the inner side panel 16 to eachother (see FIG. 1). The welding process is continuously performed alongthe welding line V (see FIG. 14), and may be one of various weldingprocesses including TIG (inert-gas tungsten-arc welding), MIG (inert-gasmetal-arc welding), laser beam welding, etc.

In step S7, the welding process is performed along the welding line V.For example, when the welding process is performed from point Q1 towardpoint Q2, as shown in FIGS. 13 and 14, an electrode 122 (or an arc orthe like) is moved along the contact region 120 where the end of theinner side panel 16 contacts the outer side panel 14. Weld beads 124 areformed along the contact region 120, thereby welding the inner sidepanel 16 and the outer side panel 14 to each other.

As the inner side panel 16 and the outer side panel 14 are welded toeach other, a welded region M is melted at a high temperature and hencebecomes deformed. If the workpiece is overly restrained during welding,then when the melted region is cooled and solidified, such deformationis not relieved, but rather causes the weld beads 124 to become strained(i.e., thermally strained) therein, and such thermally strained weldbeads 124 may possibly become cracked.

According to the welding process performed using the welding jig 10 a,when the weld beads 124 are formed due to heat and the welded region isexpanded, the outer side panel 14 is deformed and pushed outwardly, asindicated by the arrow A0 (see FIG. 13). At this time, the abutmentpoint P, which is held in attachment against the abutment 68 of theattachment 24, undergoes a force A1 depending on the arrow A0. Morespecifically, the force A1 is determined depending on the direction andsize of the arrow A0, as well as the position of the abutment point P,and is oriented substantially outwardly.

The abutment point P causes the abutment 68 to compress the spring 64under the force A1. When the force A1 is small, the compression of thespring 64 is small, and when the force A1 is large, the compression ofthe spring 64 is large.

The initial abutment point P can be displaced to a position Px, wherethe repulsive force of the compressed spring 64 and the force A1 areheld in equilibrium. Therefore, the attachment 24 functions to allow thewelded beads 124 to shrink, while absorbing the thermal strain at a hightemperature, after the inner side panel 16 and the outer side panel 14have been welded to each other. Thus, any thermal strain exerted withinthe welded beads 124 after they are cooled is very small.

Inasmuch as the distance between the abutment point P and the positionPx is very small, such a distance will not appear as a dimensionalerror.

In FIG. 13, arrow A0 represents the direction in which the welded regionis expended due to the formation of weld beads 124, wherein the arrow isillustrated as being substantially aligned with the surface of the endof the outer side panel 14. However, thermal strain may be absorbedwithout regard to the direction in which the arrow A0 is oriented. Forexample, if the arrow A0 is directed outwardly, then the spring 64 ofthe attachment 24 will be compressed, depending on the direction andsize thereof, in order to absorb the thermal strain.

If the arrow A0 is directed inwardly, then weld beads 124 are formedwhile shrinking, and the attachment 24 does not restrain the shrinkingdeformation of the weld beads 124.

In step S7, the welding point determining unit 154 determines a weldedspot M (see FIG. 14), where the welding process takes place, from datasupplied from the welding robot control unit 130 and the jig robotcontrol unit 132.

In step S8, the welding point determining unit 154 calculates a distanceL1 between the position of the welded spot M and a reference point(e.g., point Q2), and compares the distance L1 with a preset distanceL0. If the distance L0 is smaller than the distance L1, then controlreturns to step S7 to continue the welding process. If the distance L0is greater than the distance L1, e.g., if the welded spot M in FIG. 14goes beyond a point B which is spaced from point Q2 by the distance L0,then control goes to step S9.

In step S9, the welding point determining unit 154 instructs thecylinder 28 of a corresponding opening/closing mechanism 47 to open theclamp arm 26, spacing the attachment 24 away from the fuel tank 12. Forexample, when the welding process is performed toward point Q2 shown inFIG. 12, the welding point determining unit 154 instructs the firstcylinder control unit 136 (see FIG. 10), which controls the cylinder 28corresponding to point Q2, to open only the attachment 24 correspondingto point Q2.

While the outer side panel 14 is held by the attachments 24, thermalstrain can be absorbed by the springs 64. However, repulsive forces ofthe springs 64 do exert certain restraint forces, which tend to developa weak thermal strain depending thereon. By contrast, in step S9, sincethe attachment 24 is spaced from the outer side panel 14, no restraintforces are applied to the outer side panel 14, further preventingthermal strain from being produced.

If the attachment 24 is spaced from the outer side panel 14 too early,then it fails to perform its function to hold the outer side panel 14.Consequently, the distance L0 needs to be set to a suitable value.Actually, the distance L0 should preferably be set to a value that isequal to or smaller than 20 mm. If the distance L0 is set to 20 mm, thenwhen the welded spot M reaches a point that is spaced from point Q2 bythe distance L0 of 20 mm and the attachment 24 is spaced from the outerside panel 14, a location which is spaced about 25 mm from point Q2 hasalready been welded and substantially solidified. Therefore, even whenthe attachment 24 is spaced from the outer side panel 14, the outer sidepanel 14 is not unduly displaced out of position. Since the attachment24 is spaced from the outer side panel 14 only at the location where thewelding process has essentially been completed, the fuel tank 12 is notheld laterally out of balance.

In step S10, it is determined whether the welded point M has reachedpoint Q6 or not. If the welded point M has reached point Q6, then thewelding process is interrupted and goes to point Q7 in step S11. Then,control goes back to step S6, and the welding process is resumed frompoint Q7 toward point Q12.

In step S12, it is determined whether the welded point M has reachedpoint Q12 or not. If the welded point M has not reached point Q12, thencontrol goes back to step S7. If the welded point M has reached pointQ12, then control goes to step S13.

In step S13, the welding process is ended. After the welding process isended, the operator releases the joint levers 108, and removes the innerjig 20. The rods 28 b of the cylinders 28 are contracted, opening theclamp arms 26. The rod 90 of the positioning mechanism 44 is lifted, andthe welded fuel tank 12 is removed in step S14.

In steps S7 and S8, the positional relationship between the welded spotM and the reference point (e.g., point Q2) is determined in real time.However, the reference point may be related to an attitude of thewelding robot 10 c in advance, and when the welding robot 10 c assumes apredetermined attitude, the attachment 24 may be spaced from the outerside panel 14. Alternatively, the welding process may be timed from itsstart, and the attachments 24 may progressively be operated afterpredetermined periods of time.

According to the present embodiment, as described above, at hightemperatures during and after the welding process, deformation of thewelded beads 124 due to thermal strain is absorbed by the springs 64 ofthe attachments 24. Therefore, essentially no thermal strain is presentin the weld beads 124 after they are cooled. Consequently, the weldbeads 124 are prevented from cracking, resulting in an increased yield.

Since the attachments 24 can be opened and closed by the clamp arms 26,the fuel tank 12 as a workpiece can easily be attached and detached.Further, the position of the clamp arms 26 can accurately be determinedby the stoppers 26 b.

The force with which the abutments, as distal ends of the attachments24, press the outer side panel 14 can be adjusted by the adjustment nuts60 and the knobs 82.

Since the outer jig 18 comprises a positioning mechanism 44 that isinserted and fixed in the fuel inlet 36 of the fuel tank 12, the fueltank 12 can quickly and accurately be positioned with respect to theouter jig 18. The inner jig 20 also is able to position the inner sidepanel 16 of the fuel tank 12 accurately with respect to the outer sidepanel 14.

According to the present embodiment, furthermore, when the welded spot Mmoves along the welding line V, the attachments 24 are progressivelyspaced from the fuel tank 12 in the order in which they are approachedby the welded spot M. Therefore, restraint forces on the outer sidepanel 14 of the fuel tank 12 are eliminated, reducing effects of thermalstrain caused by welding, and preventing the fuel tank 12 from cracking.As a result, fuel tanks 12 can be manufactured with an increased yield.

Since the attachments 24 hold the fuel tank 12 with springs 64, theeffects of thermal strain caused by welding can be reduced even whilethe attachments 24 are holding the fuel tank 12.

Because the outer surface of the end of the outer side panel 14 and theinner surface of the end of the inner side panel 16 are superposed andheld by the attachments 24 while they are being welded, the welded fueltank 12 is free of flanges. Therefore, the fuel tank 12 canappropriately be used as the fuel tank for an American-type motorcyclethat needs to be aesthetically pleasing.

The welding jig 10 a described above comprises eight clamp arms 26.However, the number of clamp arms 26 may be increased or reduceddepending on the size and shape of the fuel tank 12. For example, a fueltank may have a total of four clamp arms 26, two each, provided in leftand right symmetrical arrays. The number of attachments 24 mounted on asingle clamp arm 26 may also be increased or reduced depending on thesize and shape of the fuel tank 12.

The welding method, the welding system, and the welding jig according tothe present invention are not limited to the above embodiment, but mayhave various arrangements and steps without departing from the scope ofthe invention.

1. A welding jig for holding a motorcycle fuel tank when the fuel tankis welded, comprising: a plurality of attachments for holding said fueltank through resilient bodies.
 2. A welding jig according to claim 1,wherein said attachments are mounted on respective arms, each of saidarms having respective opening/closing mechanisms; and when said armsare fully opened, said arms are opened wide enough to allow said fueltank to be attached and detached, and when said arms are fully closed,said arms are positioned by respective stoppers to hold said fuel tankwith said attachments.
 3. A welding jig according to claim 1, whereinsaid attachments comprise pressing force adjusters for adjusting apressing force with which said fuel tank is held.
 4. A welding jigaccording to claim 1, further comprising: an outer jig for supporting anouter side panel of said fuel tank; and an inner jig for supporting aninner side panel of said fuel tank, wherein said attachments areprovided in said outer jig, and hold side portions and/or end portionsof said outer side panel.
 5. A welding jig according to claim 4, whereinsaid outer side panel of said fuel tank comprises inwardly extendedends, wherein said attachments hold said fuel tank while outer surfacesof the ends of said outer side panel and inner surfaces of ends of saidinner side panel are superposed, or while the ends of said outer sidepanel and the ends of said inner side panel are in abutment against eachother.
 6. A welding jig according to claim 1, further comprising: apositioning mechanism for insertion into a fuel inlet defined in anupper surface of said fuel tank, and contacting an inner portion of saidfuel tank to hold said fuel tank.
 7. A welding jig according to claim 1,wherein each of said attachments has a distal end abutting against saidfuel tank, said distal end having a tilting mechanism tiltable in anydirection.